1. Field of the Invention
This invention relates to an ultrasonic motor (hereinafter also referred to as a vibration driven motor) endowed with the friction clutch function.
2. Related Background Art
In recent years, a vibration driven motor or a so-called bar-like vibration driven motor in which electrical signals differing in phase from each other are applied, for example, to piezoelectric elements as electro-mechanical energy conversion elements to thereby generate a plurality of bending vibrations differing in phase in time in different planes of a vibration member, whereby circular or elliptical motion is excited in the surface of the vibration member of a metal or the like to cause relative movement between a member (e.g. a rotor) in contact with said surface and said vibration member has been proposed, for example, in U.S. Pat. No. 4,562,374.
In this bar-like vibration driven motor, the vibration member and the rotor cannot be disposed so as to surround, for example, the photo-taking lens of a camera as in a well-known ring-like vibration driven motor. Therefore, it is unavoidable to adopt a construction in which the rotational force of the rotor is transmitted to a driven member, for example, a focusing lens through a transmission mechanism such as a gear.
However, where the driven member (for example, the lens) is driven through the transmission mechanism such as a gear with the bar-like ultrasonic motor as a drive source, if a high load is applied to a rotation output member such as a gear for transmitting the rotational force of the rotor to the driven member, the rotation of the rotor becomes unstable. In the worst case, the vibration member of the motor deviates from its resonant state and the rotation of the rotor stops. This has led to the problem that, although the vibration member is being excited, the rotor stops at that location, thereby causing the creation of sounds (noises) or the creation of abnormal abrasion between the stator (vibration member) and the rotor.
So, in order to solve such a problem, in Japanese Laid-Open Patent Application No. 2-97281, design is made such that if a high load is applied, the rotor is idly rotated. However, in this system, it is necessary to newly provide a friction plate between the rotor and the output shaft, and this could not be said to be effective in respect of space and cost.